1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems use a network of base stations to provide wireless connectivity to one or more mobile units. In some cases, the mobile units may initiate wireless communication with one or more base stations in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central element such as a Base Station Controller (BSC). The BSC may then transmit paging messages to the target mobile unit via one or more base transceiver stations or BTSs. The target mobile unit may establish a wireless link to one or more of the base transceiver stations in response to receiving the page from the wireless communication system. A radio resource management function within the BSC receives the voice and/or data and coordinates the radio and time resources used by the set of base transceiver stations to transmit the information to the target mobile unit. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of base transceiver stations.
A conventional base station provides wireless connectivity within a geographical region that is referred to as a cell, a macrocell, and/or a sector. Conventional base transceiver stations can transmit signals using a predetermined amount of available transmission power. The range of the macrocell is determined by numerous factors including the available transmission power, angular distribution of the available power, obstructions within the macrocell, environmental conditions, and the like. For example, the range of a macrocell can vary from as little as 300 m in a densely populated urban environment to as much as 10 km in a sparsely populated rural environment. The coverage area can also vary in time if any of these parameters changes.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers, that implement distributed communication network functionality. For example, each base station router may combine Radio Network Controller (RNC) and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the separate RNC and PDSN entities in hierarchical networks may be reduced or removed.
At least in part because of the reduced cost and complexity of deploying a base station router, base station routers may be deployed in locations that are impractical for conventional base stations. For example, a base station router may be deployed in a residence or building to provide wireless connectivity to the occupants of the residents of the building. Base station routers deployed in a residence are typically referred to as home base station routers or femtocells because they are intended to provide wireless connectivity to a much smaller area (e.g., a femtocell) that encompasses a residence. Femtocells have a much smaller power output than conventional base stations that are used to provide coverage to macrocells. For example, a typical femtocell has a transmission power on the order of 10 mW. Consequently, the range of a typical femtocell is much smaller than the range of a macrocell. For example, a typical range of a femtocell is about 100 m. Clusters of femtocells may also be deployed to provide coverage to larger areas and/or to more users.
Femtocells are expected to be deployed in conjunction with a macro-cellular network in an overlay configuration. For example, a macro-cellular network may be used to provide wireless connectivity to a neighborhood that includes numerous residences. Any mobile unit traveling through the neighborhood or located in one of the residences can access the wireless communication system using the macro-cellular network. Individual femtocells can be deployed in one or more of the residences to provide overlay coverage within (or near) the residence. Clusters of femtocells can also be deployed in one or more of the buildings to provide overlay coverage within (or near) the building. In either case, there will be a one-to-many relationship between the macrocells and the femtocells within the coverage area. However, mobile units will typically only be authorized to camp on selected femtocells. For example, mobile units operated by an individual user can be authorized to camp on femtocells that were installed by the user in their residence. For another example, mobile units operated by employees can be authorized to camp on femtocells in a femtocell cluster installed by a business.
In many cases, portions of the existing macro-cellular network have been in place for a period of time and may therefore be implemented using previous generation radio access technologies. Consequently, the relatively recently installed femtocells may use a more recent generation of radio access technology than the existing macrocells. For example, the wireless communication system may include second-generation macrocells and third generation femtocells. As the user moves throughout the geographic areas served by the macrocells and the femtocells, inter-radio access technology (inter-RAT) hand offs may be needed to handoff the mobile unit between the second-generation (2G) macrocells and the third generation (3G) femtocells. Conventional communication systems use radio conditions and/or the availability of radio resources to determine when to perform an inter-radio access technology hand off. For example, channel qualities and/or signal strengths can be measured using signals transmitted between the mobile unit and the macrocells and/or the femtocells. The conventional system hands off the mobile unit from a macrocell to a femtocell when the channel qualities and/or signal strengths for signals transmitted by the macrocell are poor relative to the measured channel qualities and/or signal strengths for the femtocell.
However, the conventional inter-RAT handoff criteria do not discriminate between generic femtocells, femtocells associated with particular mobile units, and macrocells. Consequently, mobile units may not be handed off to authorized home and/or business femtocells as long as the radio conditions in the macro-cellular network are sufficiently high quality, even if the user is inside the home or business covered by the associated femtocell. For example, radio conditions in the macro-cellular network may remain sufficiently high quality to prevent substantially all inter-RAT handovers to the femtocell, e.g., when the femtocell is deployed at the center of a macrocell coverage area.